If you're the serious tinkering type (as opposed to the rest of us who just like to occasionally break things), it's time to check out HTCdev.com. Two-thirds of the site, announced at the beginning of June, are now live. The OpenSense SDK gives developers easy access to take advantage of all of the HTC Sense APIs and add-ons. (For example, the tablet pen for the HTC Flyer.)
Then there's HTC's kernel source repository. HTC's been putting out kernel source for quite some time, and now it's got a new home.
And, finally, there's the much-anticipated bootloader unlock web tool. It's not live yet, but should be later this month as HTC updates the Sensation then the EVO 3D to allow for unlocked bootloaders.